Typical motor drive systems consist of a simple motor driven by an inverter as shown in FIG. 1. Safety critical aerospace applications require a certain number of redundancies designed into the system architecture and this cannot be achieved using the simplex motor drive architecture shown in FIG. 1.
These redundancies have been provided by multi-channel motor drive designs as shown, for example, in FIG. 2. FIG. 2 shows a dual channel three phase fault tolerant motor drive system. Other numbers of multiple channels can also be used.
To keep weight and size to the lowest level, permanent magnet motors are used since they have a very good torque/power density ratio in comparison to other motor drive alternatives such as switched reluctance or induction motors. The issue with a permanent magnet solution is that the magnets' field cannot be switched off under failure of either the motor or the drive, and one of the most severe failures is a motor winding, or inverter switch being shorted.
Multi-channel motor drive systems can be used in an active/active or active/standby configuration. In both cases if a failure occurs either within the motor or the drive circuit, then the remaining healthy channels need to maintain the functionality of the system.
In the event of an inverter switch becoming shorted in one of the channels or lanes, that particular channel or lane will be switched off or deactivated and one of the other channels or lanes, e.g. a lane which was in stand-by mode, will be (or will remain) activated to drive the motor.
Whilst this provides improved safety in the event of a failure, the deactivated, faulty channel or lane will, even though not selected to be the driving lane, still have some effect on the motor. Some current will continue to flow to the motor from this lane due to the motor windings inducing a voltage due to the rotor magnet rotational speed. The reason is that with a permanent magnet motor, the magnets' field cannot be switched off under failure of the drive. These voltages will induce currents which will induce drag torque.
Knowing this, designers of these multi-channel systems must design the channels such that if one fails, the channel(s) taking over as the driving channel(s) can compensate for this drag torque and associated power losses. In case of an inverter switch being shorted, the system will detect the failure, and the faulty inverter will be deactivated. By doing so the lane with the faulty inverter will not be able to produce positive torque, and the whole motor drive lane will be considered faulty. In order to maintain performance, the remaining healthy channels need to increase their torque by 1/n. This can be achieved by increasing the current in each channel by 1/n (if steel material saturation is ignored). In addition, however, the motor windings connected to the faulty inverter channel will induce a voltage due to the rotor magnet rotational speed. These voltages will induce currents, which will induce drag torque. This phenomenon is illustrated in FIG. 3 for a two three-phase channel motor drive system for the condition where the induced voltage of phase b is greater than that of phase a, but the idea can be generalised for any n three-phase channel system. The circulating currents will induce a drag torque that the remaining healthy channels need to overcome. FIG. 4 shows the drag torque produced from the shorted switch, and the power loss in the motor windings. This design consideration means that the drive channels have to be greater in size than if they did not have to compensate the drag torque.
The present invention aims to provide a multi-channel motor drive system for a permanent magnet motor that actually minimises drag torque when a faulty lane is deactivated, meaning that the steps needed to compensate for drag torque, in the activate lane, are less onerous and the lanes can, therefore, be smaller in size.